Process for manufacturing alkanes by reacting other alkanes with methane

ABSTRACT

The present invention relates to a process for the manufacture of alkanes comprising a catalytic reaction resulting from contacting methane with at least one other starting alkane (I) in the presence of a metal compound (C) capable of catalysing a reaction for the splitting and/or recombination of a carbon-carbon bond and/or of a carbon-hydrogen bond and/or of a carbon-metal bond, which catalytic reaction results in the formation of at least one final alkane (II) having a number of carbon atoms equal to or greater than (2). In the process, the contacting operation is carried out under a methane partial pressure equal to or greater than 0.1 MPa, preferably in the range from 0.1 to 100 MPa. The metal compound (C) can be chosen from metal compounds supported on and dispersed over a solid support, metal compounds supported on and grafted to a solid support and non-supported metal compounds. Under these conditions, it was found that the yield of the catalytic reaction was improved, and that the catalytic stability and activity of the metal compound (C) over timer were greatly enhanced.

This application is the U.S. National Phase of International ApplicationPCT/GB03/00257, filed 22 Jan. 2003, which designated the U.S.

The present invention relates to an improved process for the manufactureof alkanes by catalytic reaction employing methane with at least oneother alkane.

Alkanes, such as methane, are generally products which are difficult toemploy in reactions because of their high chemical inertia and are usedessentially as fuels and energetic materials. Furthermore, methane isknown to be one of the most widespread sources of hydrocarbons in theworld.

It is already known to convert alkanes into other alkanes, for exampleby hydrogenolysis reactions, which consist of reactions for thesplitting or opening of a carbon-carbon bond by hydrogen. Isomerizationreactions which convert an alkane into one of its isomers, for examplen-butane into isobutane, are also known. These reactions are generallycarried out at relatively high temperatures and in the presence ofcatalysts based on metals, in particular on transition metals,especially dispersed or fixed to solid supports, for example to metaloxides or refractory oxides. More particularly, the catalysts can becatalysts of nickel black, Ni/SiO₂, platinum black, Pt/SiO₂ orPd/Al₂O₃type or films of tungsten or of rhodium optionally mixed withcopper, tin or silver. In some cases, it has been possible tosimultaneously observe reactions for the homologation of alkanes, whichconsist of reactions in which initial (or starting) alkanes areconverted into higher homologous alkanes. However, these homologationreactions often remain very minor in comparison with the hydrogenolysisor isomerization reactions and their performances are generally verypoor.

Nevertheless, it remains the case that a process for the conversion ofan alkane into one of its homologues would constitute a means forenhancing in value alkanes in general and in particular methane. It isknown that light alkanes and methane in particular are difficult toenhance in value in the chemical or petrochemical industry, whereasheavier alkanes often have greater commercial interest, in particular asadditives intended to increase the octane number of fuels, or asstarting materials in thermal or catalytic cracking reactions formanufacturing, for example, olefins or dienes.

In this sense, International Patent Application WO 01/04077 discloses aprocess for the manufacture of alkanes comprising a reaction resultingfrom bringing methane into contact with at least one other startingalkane (A) in the presence of a metal compound capable of catalysing ametathesis of alkanes. The reaction results in the formation of at leastone or two final alkanes (B) having a number of carbon atoms lower thanor equal to that of the starting alkane (A) and at least equal to 2, forexample according to the following equation.

in which n is an integer at least equal to 2 and a is an integer rangingfrom 1 to n−1.

It is specified in particular that the reaction can be carried out at atemperature ranging from −30 to +400° C. and under an absolute pressurewhich can vary within a very wide range, preferably under an absolutepressure of less than atmospheric pressure, and in particular under amethane partial pressure equal to 0.0645 MPa, as shown in the examples.

It has been observed that the process for the manufacture of alkanesdescribed above may have certain disadvantages during itsimplementation. In particular, it has been noticed that, on applying amethane partial pressure of less than atmospheric pressure, the yield ofthe reaction resulting from bringing methane into contact with anotheralkane very rapidly decreases over time. The fact has also beendemonstrated that, under these conditions, the metal compound used ascatalyst rapidly loses its activity and is very unstable over time.

A process for the manufacture of alkanes has now been found which makesit possible to considerably improve the yield of the reaction betweenmethane and the other starting alkane or alkanes, in particular theyield of the reaction with respect to the metal compound employed ascatalyst, or, in other words, to considerably increase the overallconversion of the starting alkane or alkanes charged to the reactionwith methane. In particular, reaction conditions have been found whichmake it possible to maintain the catalytic activity of the metalcompound at a particularly high level over time and consequently toincrease the overall catalytic activity of the reaction. It has thusbeen observed with surprise that, on carrying out this reaction under ahigh methane partial pressure, the metal compound used as catalystexhibits a considerably increased catalytic stability over time, withoutit being possible to find an immediate explanation for this phenomenon.

A particular subject-matter of the invention is a process for themanufacture of alkanes comprising, particularly as a main stage, acatalytic reaction resulting from bringing methane into contact with atleast one other starting alkane (I) in the presence of a metal compound(C) capable of catalysing a reaction for the splitting and/orrecombination of a carbon-carbon bond and/or of a carbon-hydrogen bondand/or of a carbon-metal bond, which catalytic reaction results in theformation of at least one final alkane (II) having a number of carbonatoms equal to or greater than 2, which process is characterized in thatthe operation is carried out under a methane partial pressure equal toor greater than 0.1 MPa, preferably equal to or greater than 0.2 MPa, inparticular equal to or greater than 0.3 or equal to or greater than 0.5MPa, and which can be chosen within a range more particularly from 0.1to 100 MPa.

The “methane-olysis” catalytic reaction, as shown by equation (1),results from bringing methane into contact with at least one otherstarting alkane (I) having n carbon atoms with n being equal to at least2, preferably to at least 3, so that the catalytic reaction generallyresults in the formation of at least one final alkane (II) or of atleast two final alkanes (II) having a number of carbon atoms rangingfrom 2 to (n−1) or even to a value greater than (n−1). This is becausethe alkane or alkanes resulting directly from the “methane-olysis”reaction can themselves participate in at least one reaction for themetathesis of other alkanes. The catalytic reaction can be writtenaccording to one or more equations (1) described above in which n is aninteger at least equal to 2, preferably at least equal to 3, and a is aninteger ranging from 1 to n−1 or even more. It is essentially anequilibrated catalytic reaction.

According to another aspect, another subject-matter of the invention isin particular a process for increasing the catalytic activity andstability of a metal compound (C) capable of catalysing a reaction forthe splitting and/or recombination of a carbon-carbon bond and/or of acarbon-hydrogen bond and/or of a carbon-metal bond, which compound isemployed, particularly as a main stage, in a catalytic reaction whichresults from bringing methane into contact with at least one otherstarting alkane (I) and which results in the formation of at least onefinal alkane (II) having a number of carbon atoms equal to or greaterthan 2, which process is characterized in that the contacting operationis carried out under a methane partial pressure equal to or greater than0.1 MPa, preferably equal to or greater than 0.2 MPa, in particularequal to or greater than 0.3 or equal to or greater than 0.5 MPa, andwhich can be chosen within a range more particularly from 0.1 to 100MPa.

It is particularly surprising to find that the application of a highmethane partial pressure in such a process has so great an effect on thecatalytic stability of the metal compound (C) when the latter is used in“methane-olysis” reactions between methane and at least one otheralkane.

According to the invention, the methane partial pressure applied whenmethane is brought into contact with at least one other starting alkane(I) can be from 0.1 to 100 MPa, preferably from 0.1 to 50 MPa, inparticular from 0.1 to 30 MPa or from 0.2 to 20 MPa, especially from 0.3to less than 10 MPa, for example from 0.3 to 9.5 MPa, or from 0.5 to 9.5MPa.

The contacting operation can be carried out at a temperature rangingfrom −30 to +500° C., preferably from 0 to 500° C. or from 0 to 400° C.,in particular from 20 to 500° C. or from 20 to 300° C. A range oftemperature particularly from 50 to 500° C., preferably from 50 to 450°C., especially from 100 to 450° C. may be preferred.

The contacting operation can be carried out by adding methane or theother starting alkane or alkanes (I) to the metal compound (C), eitherseparately and in any order, or simultaneously with at least twoseparate introductions, or mixed beforehand and using, in this case, asingle introduction.

Methane and the starting alkane or alkanes (I) can be employed in amolar ratio of methane to the starting alkane or alkanes (I) rangingfrom 0.1:1 to 10⁵:1, preferably from 1:1 to 10⁴:1, in particular from1:1 to 5×10³:1, for example from 1:1 to 3×10³:1. During the contactingoperation, it is particularly advantageous to employ a molar amount ofmethane greater than or even much greater than the molar amount of theother starting alkane or alkanes (I). Thus, for example, the methanestarting alkane(s) (I) molar ratio can range from 60:1 to 10⁵:1,preferably from 60:1 to 10⁴:1, in particular from 60:1 to 5×10³:1.

In a “methane-olysis” reaction carried out in particular batchwise, themetal compound (C) can be employed when methane is brought into contactwith the other starting alkane or alkanes (I) in a molar ratio ofmethane to the metal of the metal compound (C) ranging from 10:1 to10⁵:1, preferably from 50:1 to 10⁴:1, in particular from 50:1 to 10³:1.

The contacting operation can be carried out in the presence of one ormore liquid or gaseous inert agents, in particular inert gases, such asnitrogen, helium or argon.

The process for the manufacture of alkanes resulting from the contactingoperation according to the invention can be carried out batchwise or,preferably, continuously. It can be carried out in the gas phase, inparticular in a mechanically stirred and/or fluidized bed reactor or astationary or circulating bed reactor, the bed being composedessentially of the metal compound (C), for example a metal compoundsupported on and dispersed over a solid support, or a metal compoundsupported on and grafted to a solid support, or a non-supported metalcompound in the form of a solid. The process can also be carried out inthe liquid phase preferably being composed essentially of the startingalkane or alkanes (I) in the liquid state, under the conditions of thecatalytic reaction. The metal compound (C) can in particular besuspended in the liquid phase, in particular when the metal compound (C)is a metal compound supported on and dispersed over a solid support or ametal compound supported on and grafted to a solid support, or anon-supported solid metal compound. It is preferable to carry out theprocess continuously, either in the gas phase or in the liquid phase, ina reaction region into which methane and the starting alkane or alkanes(I) are introduced continuously and where they come into contact withthe metal compound (C) present in the said region, the productsresulting from the contacting operation being withdrawn continuouslyfrom the region in order to be partially or completely separated fromthe starting materials introduced and optionally to be recycled to theregion.

The contacting operation of the process according to the inventioncomprises the use of methane with at least one starting alkane (I) whichcan be a substituted or unsubstituted acyclic alkane, that is to saycomposed of a linear or branched, but unclosed, carbonaceous chain. Itcan correspond to the general formula:C_(n)H_(2n+2)   (2)in which n is an integer ranging from 2 to 60 or from 3 to 60,preferably from 3 to 50, in particular from 3 to 20.

The starting alkane (I) can also be a substituted (or branched) cyclicalkane, that is to say composed of a branched and closed carbonaceouschain. The substituted cyclic alkane comprises at least one substitution(or branching) itself composed in particular of a linear or branchedcarbonaceous chain, for example of an alkyl chain. It can correspond tothe general formula:C_(n)H_(2n)   (3)in which n is an integer ranging from 5 to 60, preferably from 5 to 20,in particular from 5 to 10.

More particularly, the starting alkane (I) can be chosen from linear orbranched C₃ to C₁₀ or C₃ to C₁₇ alkanes, for example from propane,n-butane, isobutane, n-pentane, isopentane, n-hexane, n-heptane,n-octane, n-nonoane and n-decane.

Thus, for example, in the process of the present invention, methane canbe brought into contact with propane, and ethane or possibly otherhigher alkanes can be formed; or else methane can be brought intocontact with n-butane, and ethane, or in particular a mixture of ethaneand of propane, and possibly other higher alkanes resulting from other“methane-olysis” and/or metathesis reactions, in particular of theethane formed and/or of the propane formed, can be formed.

The starting alkane (I) can also be chosen from paraffins, such asn-paraffins, isoparaffins and cycloparaffins, for example linear,branched or cyclic C₁₈ to C₆₀ or C₂₂ to C₆₀ or C₁₈ to C₄₅ alkanes.

The process of the present invention can be carried out by contactingmethane with one or more starting alkanes (I), that is to say a mixtureof two or more starting alkanes (I), such as those described above. Whenone starting alkane (I) is used with methane, the process resulting fromsuch a contacting generally involves an alkane “methane-olysis” reactionsimultaneously with an alkane self-metathesis reaction. When two or morestarting alkanes (I) are used with methane, the process resulting fromsuch a contacting may involve alkane “methane-olysis” reactionssimultaneously with some alkane metathesis reactions and alkaneself-metathesis reactions.

Methane is brought into contact with at least one starting alkane (I) inthe presence of a metal compound (C) which is capable of catalysing areaction for the splitting and/or recombination of a carbon-carbon bondand/or of a carbon-hydrogen bond and/or of a carbon-metal bond. Themetal compound (C) can be chosen from metal compounds supported on anddispersed over a solid support, metal compounds supported on and graftedto a solid support, and non-supported metal compounds.

The term “metal compound” is understood to mean generally both a metaland a chemical compound comprising a metal, that is to say a metalbonded chemically to at least one other element.

The term “metal compound (or atom) grafted to a solid support” isunderstood to mean generally a metal compound (or atom) which is(chemically) fixed to the support, in particular by at least one singleor multiple bond, and which in particular is bonded directly to at leastone of the essential elements (or constituents) of the solid support.

The Periodic Table of the Elements cited below is that proposed by theIUPAC in 1991 and which is found, for example, in “CRC Handbook ofChemistry and Physics”, 76th Edition (1995-1996), by David R. Lide,published by CRC Press, Inc. (USA).

The metal atom, Me, present in the metal compound (C) can be at leastone metal chosen from the lanthanides, the actinides and the metals fromGroups 2 to 12, preferably the transition metals from Groups 3 to 12, inparticular from Groups 3 to 10, of the Periodic Table of the Elements.The metal atom, Me, can in particular be at least one metal chosen fromscandium, yttrium, lanthanum, titanium, zirconium, hafnium, vanadium,niobium, tantalum, chromium, molybdenum, tungsten, ruthenium, palladium,platinum, iridium, cerium and neodymium. It can preferably be chosenfrom titanium, zirconium, hafnium, vanadium, niobium, tantalum,chromium, molybdenum and tungsten, and more preferably from tantalum,chromium, yttrium, vanadium, niobium, molybdenum and tungsten.

The metal compound (C) can in particular be chosen from metal compoundssupported and dispersed on a solid support, that is to say metalcompounds or metals supported on and dispersed over a support, such as,for example, metal particles dispersed over a support. The metalcompounds supported on and dispersed over a support can compriseidentical different metal atoms, Me, that is to say metal atoms whichare not fixed (chemically) to the support, or, in other words, whichhave no connection via single or multiple bonds with the support, inparticular with the essential elements of the support.

The metal compound (C) can also be chosen from metal compounds supportedon and grafted to a solid support, comprising one or more identical ordifferent metal atoms, Me, in particular fixed (chemically) to thesupport, especially via single or multiple bonds. The metal atoms, Me,grafted to the support can additionally be advantageously bonded to atleast one hydrogen atom and/or to at least one hydrocarbon radical.

In the case where the metal atom, Me, grafted to a support isadditionally bonded to at least one hydrogen atom, the metal compound(C) can be chosen from supported and grafted metal compounds comprisinga solid support to which is grafted at least one metal hydride, inparticular a hydride of the metal Me.

In the case where the metal atom, Me, grafted to a support isadditionally bonded to at least one hydrocarbon radical, the metalcompound (C) can be chosen from supported and grafted metal compoundscomprising a solid support to which is grafted at least oneorganometallic compound, in particular an organometallic compound of themetal Me.

Thus, the metal compound (C) can advantageously be chosen from metalhydrides and/or organometallic compounds, in particular of the metal Me,supported on and grafted to a solid support.

The metal compound (C) can also advantageously be chosen from supportedand grafted metal compounds comprising a solid support to which aregrafted at least two types of metal atom, Me, one in a form (A) of ametal compound where the metal atom is bonded to at least one hydrogenatom and/or to at least one hydrocarbon radical, and the other in a form(B) of a metal compound where the metal atom is solely bonded to thesupport and optionally to at least one other component which is neithera hydrogen atom nor a hydrocarbon radical. The metal atoms, Me, cancorrespond to identical or different metals for each of the forms (A) or(B). The metal atoms, Me, present in the form (A) can be identical to ordifferent from those present in the form (B). When the forms (A) and (B)coexist in the metal compound (C), the degree of oxidation of the metalatoms, Me, present in the form (A) can be identical to or different fromthat of the metal atoms, Me, present in the form (B).

The solid support can be any solid support, in particular essentiallycomprising atoms M and X which are different from one another and whichare generally bonded to one another via single or multiple bonds, so asto form in particular the molecular structure of the solid support. Theterm “support essentially comprising atoms M and X” is understood tomean generally a support which comprises, as predominant constituents,the atoms M and X and which can additionally comprise one or more otheratoms capable of modifying the structure of the support.

The atom M of the support can be at least one of the elements chosenfrom the lanthanides, the actinides and the elements from Groups 2 to 15of the Periodic Table of the Elements. The atom M of the support can beidentical to or different from the metal atom, Me. The atom M can be atleast one of the elements chosen in particular from magnesium, titanium,zirconium, cerium, vanadium, niobium, tantalum, chromium, molybdenum,tungsten, boron, aluminium, gallium, silicon, germanium, phosphorus andbismuth. The atom M of the support is preferably at least one of theelements chosen from the lanthanides, the actinides and the elementsfrom Groups 2 to 6 and from Groups 13 to 15 of the Periodic Table of theElements, in particular from silicon, aluminium and phosphorus.

The atom X of the support, which is different from the atom M, can bechosen from at least one of the elements from Groups 15 and 16 of thePeriodic Table of the Elements, it being possible for the element to bealone or itself optionally bonded to another atom or to a group ofatoms. In the case where the atom X of the support is chosen inparticular from at least one of the elements from Group 15, it canoptionally be bonded to another atom or to a group of atoms chosen, forexample, from a hydrogen atom, a halogen atom, in particular a fluorine,chlorine or bromine atom, a saturated or unsaturated hydrocarbonradical, a hydroxyl group of the formula (—OH), a hydrogensulphide groupof formula (—SH), alkoxide groups, thiolate groups, or silylated (orsilane) or organosilylated (or organosilane) groups. The atom X of thesupport is preferably at least one of the elements chosen from oxygen,sulphur and nitrogen and more particularly from oxygen and sulphur.

The atoms M and X which represent generally the essential elements ofthe solid support can in particular be bonded to one another via singleor double bonds. In a preferred alternative form, the solid support canbe chosen from oxides, sulphides and azides in particular of M, and themixtures of two or three of the oxides, sulphides and/or azides. Moreparticularly, the support can be an oxide in particular of M and can bechosen from simple or mixed oxides in particular of M, or mixtures ofoxides in particular of M. The support can be, for example, chosen frommetal oxides, refractory oxides and molecular sieves, in particular fromsilica, alumina, aluminosilicates, aluminium silicates, simple ormodified by other metals, zeolites, clays, titanium oxide, cerium oxide,magnesium oxide, niobium oxide, tantalum oxide and zirconium oxide. Thesupport can also be a metal oxide or refractory oxide, optionallymodified by an acid, and optionally comprising in particular an atom Mbonded to at least to atoms X which are different from one another, forexample the oxygen atom and the sulphur atom. Thus, the solid supportcan be chosen from sulphated metal oxides or refractory oxides, forexample a sulphated alumina or a sulphated zirconia. The support canalso be chosen from metal sulphides or refractory sulphides andsulphided metal oxides or refractory oxides, for example a molybdenumsulphide, a tungsten sulphide or a sulphided alumina. The support canalso be chosen from azides, in particular boron azide.

The essential constituents of the solid support are preferably the atomsM and X described above. In addition, the solid support has theadvantage of generally exhibiting, at the surface, atoms X capable offorming part of the coordination sphere of the metal atoms, Me, of themetal compound (C), in particular when the latter is chosen from themetal compounds supported on and grafted to a solid support. Thus, atthe surface of the support, the atom X which is bonded to at least onemetal atom, Me, can advantageously be bonded to at least one atom M. Thebonds between X and M, on the one hand, and between X and Me, on theother hand, can be single or double bonds.

In the case of a metal compound supported on and grafted to a support,the metal atom, Me, in particular present in the form (A), can bebonded, on the one hand, to the support, in particular to at least oneatom constituting the support, preferably the atom X of the support asdescribed above, and in particular via a single or double bond, and, onthe other hand, to at least one hydrogen and/or to at least onehydrocarbon radical, R, in particular via a carbon-metal single, doubleor triple bond. The hydrocarbon radical, R, can be saturated orunsaturated, can have from 1 to 20, preferably from 1 to 10, carbonatoms and can be chosen from alkyl, alkylidene or alkylidyne radicals,in particular C₁ to C₁₀, preferably C₁, alkyl, alkylidene or alkylidyneradicals, aryl radicals, in particular C₆ to C₁₀ aryl radicals, andaralkyl, aralkylidene or aralkylidyne radicals, in particular C₇ to C₁₄aralkyl, aralkylidene or aralkylidyne radicals.

In the case of a metal compound supported on and grafted to a support,the metal atom, Me, in particular present in the form (A) can be bondedto the hydrocarbon radical, R, via one or more carbon-metal single,double or triple bonds. It can be a carbon-metal single bond, inparticular of the σ type: in this case, the hydrocarbon radical, R, canbe an alkyl radical, in particular a linear or branched alkyl radical,for example a C₁ to C₁₀, preferably C₁, alkyl radical, or an arylradical, for example the phenyl radical, or an aralkyl radical, forexample the benzyl radical. The term “alkyl radical” is understood tomean generally a monovalent aliphatic radical originating from theremoval of a hydrogen atom in the molecule of an alkane or of an alkeneor of an alkyne, for example the methyl, ethyl, propyl, neopentyl, allylor ethynyl radical. The methyl radical is preferred.

It can also be a carbon-metal double bond, in particular of the π type:in this case the hydrocarbon radical, R, can be alkylidene radical, inparticular a linear or branched alkylidene radical, for example a C₁ toC₁₀, preferably C₁, alkylidene radical, or an aralkylidene radical, forexample C₇ to C₁₄ aralkylidene radical. The term “alkylidene radical” isunderstood to mean generally a bivalent aliphatic radical originatingfrom the removal of two hydrogen atoms on the same carbon from themolecule of an alkane or an alkene or of an alkyne, for example, themethylidene, ethylidene, propylidene, neopentylidene or allylideneradical. The methylidene radical is preferred. The term “aralkylideneradical” is understood to mean generally a bivalent aliphatic radicaloriginating from the removal of two hydrogen atoms on the same carbonfrom an alkyl, alkenyl or alkynyl linking unit of an aromatichydrocarbon.

It can also be a carbon-metal triple bond: in this case, the hydrocarbonradical, R, can be an alkylidyne radical, in particular a linear orbranched alkylidine radical, for example a C₁ to C₁₀, preferably C₁,alkylidyne radical, or an aralkylidyne radical, for example a C₇ to C₁₄aralkylidyne radical. The term “alkylidyne radical” is understood tomean generally a trivalent aliphatic radical originating from theremoval of three hydrogen atoms on the same carbon from the molecule ofan alkane or of an alkene or of an alkyne, for example the methylidyne,ethylidyne, propylidyne, neopentylidyne or allylidyne radical. Themethylidyne radical is preferred. The term “aralkylidyne radical” isunderstood to mean generally a trivalent aliphatic radical originatingfrom the removal of three hydrogen atoms on the same carbon from analkyl, alkenyl or alkynyl linking unit of an aromatic hydrocarbon.

The metal compound (C) can advantageously be chosen from the metalcompounds supported on and grafted to a solid support comprising themetal atom, Me, present in the two forms (A) and (B). Such a compoundhas the advantage of exhibiting a particularly high catalytic activityin reactions for the splitting and/or recombination of a carbon-carbonbond and/or a carbon-hydrogen bond and/or a carbon-metal bond. The form(A) of the metal compound is that described above. In the form (B), themetal atom, Me, is preferably bonded solely to the support, inparticular to one or more atoms constituting the essential elements ofthe support, in particular to one or more atoms X of the support asdescribed above, for example via single or double bonds.

In the form (B), the metal atom, Me, can optionally be bonded, inaddition to the support, to at least one other component which isneither a hydrogen atom nor a hydrocarbon radical. The other componentbonded to the metal Me can be, for example, at least one of the elementsfrom Groups 15 to 17 of the Periodic Table of the Elements, whichelement can be alone or itself bonded to at least one hydrogen atomand/or to at least one hydrocarbon radical and/or to at least onesilylated (or silane) or organosilylated (or organosilane) group. Inparticular, the metal atom, Me, present in the form (B) can optionallybe bonded, in addition to the support, to at least one atom of theelements chosen from oxygen, sulphur, nitrogen and halogens, inparticular flourine, chlorine or bromine. Thus, for example, the metalatom, Me, can be bonded, via a single bond, to one or more halogenatoms, in particular fluorine, chlorine or bromine. It can also bebonded, via a double bond, to one or more oxygen or sulphur atoms, inparticular in the form of a metal oxide or a metal sulphide. It can alsobe bonded, via a single bond, to at least one oxygen or sulphur atom,itself bonded to a hydrogen atom or to a saturated or unsaturatedhydrocarbon radical, in particular a C₁ to C₂₀, preferably C₁ to C₁₀,radical, for example in the form of a hydroxide, of a hydrogensulphide,of an alkoxide or of a thiolate. It can also be bonded, via a singlebond, to a silylated or organosilylated group. It can also be bonded,via a single bond, to an amido (or amide) group, for example of formulae(NH₂—), (NHR—) or (NRR′—), in which R and R′, which are identical ordifferent, represent saturated or unsaturated hydrocarbon radicals, inparticular C₁ to C₂₀, preferably C₁ to C₁₀, radicals, or silylated ororganosilylated groups, or else can be bonded, via a double bond, to animide (or imide) group, for example of formula (NH≡) or via a triplebond, to a nitride, (or azide) group for example of formula (NH≡).

It is preferable to use the metal compounds supported on and grafted toa solid support in which the metal atoms, Me, grafted to the supportexist simultaneously in the two forms (A) and (B), because these metalcompounds advantageously exhibit a very high catalytic activity in thesplitting and/or recombination reactions described above, in particularwhen, per 100 mol of the metal Me grafted to the support, the metalcompound comprises:

(a) from 5 to 95 mol, preferably from 10 to 90 mol, in particular from20 to 90 mol, especially from 25 to 90 mol, or more particularly from 30to 90 mol, of the metal Me in the form (A), and

(b) from 95 to 5 mol, preferably from 90 to 10 mol, in particular from80 to 10 mol, especially from 75 to 10 mol, or more particularly, from70 to 10 mol, of the metal Me in the form (B).

The metal compound (C) can also be chosen from the non-supported metalcompounds, that is to say non-supported metal compounds or non-supportedmetals, which can in particular be provided in any solid form,preferably in the form of films or of particles.

The metal compounds (C) described above can be prepared in various ways.A first process for the preparation of a metal compound supported on andgrafted to a solid support can comprise the following stages:

(a) grafting an organometallic precursor (P) comprising the metal Me,bonded to at least one hydrocarbon ligand, to the solid support, and

(b) treating the solid product resulting from stage (a) with hydrogen ora reducing agent capable of forming a metal Me-hydrogen bond, preferablyby hydrogenolysis of the hydrocarbon ligands, at a temperature inparticular at most equal to the temperature T1 at which the metalcompound is formed solely in the form (A) as defined above.

The temperature of stage (b) is chosen in particular so that it is atmost equal to the temperature T1 where only the form (A) of the metalcompound is formed, that is to say where only the metal hydride isformed. The temperature of stage (b) can in particular be chosen withinthe range from 50 to 160° C., preferably from 100 to 150° C. Stage (b)can take place under an absolute pressure of 10⁻³ to 10 MPa and for aperiod of time which can range from 1 to 24 hours, preferably from 5 to20 hours.

A second process for the preparation of a metal compound supported onand grafted to a solid support can comprise the following stages:

(a) grafting an organometallic precursor (P) comprising the metal Me,bonded to at least one hydrocarbon ligand, to the solid support, and

(b) treating the solid product resulting from stage (a) with hydrogen ora reducing agent capable of forming a metal Me-hydrogen bond, preferablyby hydrogenolysis of the hydrocarbon ligands, at a temperature greaterthan the temperature T1 at which the metal compound is formed solely inthe form (A), and lower than the temperature T2 at which the metalcompound is formed solely in the form (B), the forms (A) and (B) beingthose described above.

The temperature of stage (b) is chosen in particular so that it isgreater than the temperature T1 where only the form (A) is formed. Itcan in particular be at least 10° C., preferably at least 20° C., inparticular at least 30° C. or even at least 50° C., greater than thetemperature T1. In addition, it is chosen in particular so that it islower than the temperature T2 where only the form (B) is formed. It canin particular be at least 10° C., preferably at least 20° C., inparticular at least 30° C. or even at least 50° C., lower than thetemperature T2. The temperature of stage (b) can, for example, be chosenwithin the range from 165° C. to 450° C., preferably from 170 to 430°C., in particular from 180 to 390° C., especially from 190 to 350° C. orfrom 200 to 320° C. Stage (b) can take place under an absolute pressureof 10⁻³ to 10 MPa, and for a period of time which can range from 1 to 24hours, preferably from 5 to 20 hours.

A third process for the preparation of a metal compound supported on andgrafted to a solid support can comprise the following stages:

(a) grafting an organometallic precursor (P) comprising the metal Me,bonded to at least one hydrocarbon ligand, to a solid support, then

(b) treating the solid product resulting from stage (a) with hydrogen ora reducing agent capable of forming a metal Me-hydrogen bond, preferablyby complete hydrogenolysis of the hydrocarbon ligands, at a temperaturein particular at most equal to the temperature T1 at which the metalcompound is formed solely in the form (A) as defined above, so as toform a metal hydride in the form (A), and

(c) heat-treating the solid, product resulting from stage (b),preferably in the presence of hydrogen or of a reducing agent, at atemperature greater than the temperature of stage (b) and lower than thetemperature T2 at which the metal compound is formed solely in the form(B) as defined above.

Stage (b) of the process can be carried out under the same conditions,in particular of temperature, as those of stage (b) of the firstpreparation process. Stage (c) can be carried out at a temperature,under a pressure and for a period of time equivalent to those describedin stage (b) of the second preparation process.

A fourth process for the preparation of a metal compound supported onand grafted to a solid support can comprise the following stages:

(a) grafting an organometallic precursor (P) comprising the metal Me,bonded to at least one hydrocarbon ligand, to the solid supportcomprising functional groups capable of grafting the precursor (P), bybringing the precursor (P) into contact with the solid support, so as tograft the precursor (P) to the support by reaction of (P) with a portionof the functional groups of the support, preferably from 5 to 95% of thefunctional groups of the support, then

(b) heat-treating the solid product resulting from stage (a), preferablyin the presence of hydrogen or of a reducing agent, at a temperatureequal to or greater than the temperature T2 at which the metal compoundis formed solely in the form (B) as defined above, then

(c) grafting, to the solid product resulting from stage (b), anorganometallic precursor (P′), identical to or different from (P),comprising the metal Me, bonded to at least one hydrocarbon ligand, themetal Me and the ligand being identical to or different from those of(P), by bringing the precursor (P′) into contact with the solid productresulting from stage (b), so as to graft the precursor (P′) to thesupport by reaction of (P′) with the remaining functional groups in thesupport, and optionally

(d) treating the solid product resulting from stage (c) with hydrogen ora reducing agent capable of forming metal Me-hydrogen bonds, preferablyby complete hydrogenolysis of the hydrocarbon ligands of the graftedprecursor (P′), at a temperature in particular at most equal to thetemperature T1 at which the metal compound is formed solely in the form(A) as defined above.

Stage (b) of the process can be carried out at a temperature such thatmost, preferably all, of the precursor (P) grafted to the support isconverted into the metal compound in the form (B). The temperatureduring stage (b) can be chosen within the range from 460° C., preferablyfrom 480° C., in particular from 500° C., up to a temperature lower thanthe sintering temperature of the support. Stage (d) is optional and canbe carried out at a temperature equivalent to that of stage (b) of thefirst preparation process.

A fifth process for the preparation of a metal compound supported on andgrafted to a solid support can comprise the following stages:

(a) grafting an organometallic precursor to the solid support under thesame conditions as in stage (a) of the preceding preparation process,then

(b) treating the solid product resulting from stage (a) under the sameconditions as in stage (b) of the preceding preparation process, then

(c) bringing the solid product resulting from stage (b) into contactwith at least one compound Y capable of reacting with the metal Me ofthe form (A) and/or (B) prepared above, the contacting operationpreferably being followed by removal of the unreacted compound Y and/orby a heat treatment at a temperature lower than the sinteringtemperature of the support, then

(d) grafting to the solid product resulting from stage (c), anorganometallic precursor (P′), identical to or different from (P),comprising the metal Me bonded to at least one hydrocarbon ligand, themetal Me and the ligand being identical to or different from those of(P), by bringing the precursor (P′) into contact with the productresulting from stage (c), so as to graft the precursor (P′) to thesupport by reaction of (P′) with the remaining functional groups in thesupport, and optionally

(e) treating the solid product resulting from stage (d) with hydrogen ora reducing agent capable of forming metal Me-hydrogen bonds, preferablyby complete hydrogenolysis of the hydrocarbon ligands of the graftedprecursor (P′), at a temperature in particular at most equal to thetemperature T1 at which the metal compound is formed solely in the form(A) as defined above.

Stage (b) of the process can be carried out at a temperature equivalentto that of stage (b) of the fourth preparation process. In stage (c),the compound Y can be chosen from molecular oxygen, water, hydrogensulphide, ammonia, an alcohol, in particular a C₁ to C₂₀, preferably C₁to C₁₀, alcohol, a thiol, in particular a C₁ to C₂₀, preferably C₁ toC₁₀, thiol, a primary or secondary C₁ to C₂₀, preferably C₁ to C₁₀,amine, a molecular halogen, in particular molecular fluorine, chlorineor bromine, and a hydrogen halide, for example of formula HF, HCl orHBr. The heat treatment optionally carried out at the end of stage (c)can be carried out at a temperature ranging from 25 to 500° C. Stage (e)is optional and can be carried out at a temperature equivalent to thatof stage (b) of the first preparation process.

In the processes for the preparation of a supported and grafted metalcompound such as those described above, the operation of grafting to asolid support employs at least one organometallic precursor (P) or (P′)comprising the metal Me bonded to at least one hydrocarbon ligand. Theprecursor can correspond to the general formula:MeR″_(a)   (4)in which Me has the same definition as above, R″ represents one or moreidentical or different and saturated or unsaturated hydrocarbon ligands,in particular aliphatic or alicylic ligands, in particular C₁ to C₂₀,preferably C₁ to C₁₀, ligands having, for example, the same definitionas that given above for the hydrocarbon radical, R, of the metalcompound (C), and a is an integer equal to the degree of oxidation ofthe metal Me. The radical R″ can be chosen from alkyly, alkylidene,alkylidyne, aryl, aralkyl, aralkylidene and aralkylidyne radicals. Themetal Me can be bonded to one or more carbons of the hydrocarbonligands, R″, in particular via carbon-metal single, double or triplebonds, such as those connecting the metal Me to the hydrocarbon radical,R, in the metal compound (C).

In the processes for the preparation of a supported and grafted metalcompound such as those described above, the solid support is preferablysubjected beforehand to a dehydration and/or dehydroxylationheat-treatment, in particular at a temperature lower than the sinteringtemperature of the support, preferably at a temperature ranging from 200to 1000° C., preferably from 300 to 800° C., for a period of time whichcan range from 1 to 48 hours, preferably from 5 to 24 hours. Thetemperature and the period of time can be chosen so as to create and/orto allow to remain, in the support and at predetermined concentrations,functional groups capable of grafting the precursor (P) or (P′) byreaction. Mention may be made, among the functional groups known for thesupports, of groups of formulae XH in which H represents a hydrogen atomand X corresponds to the same definition as given above for the supportand in particular can represent an atom chosen from oxygen, sulphur andnitrogen. The most well-known functional group is the hydroxyl group.

The grafting operation in general can be carried out by sublimation orby bringing the precursor into contact in a liquid medium or insolution. In the case of sublimation, the precursor used in the solidstate can be heated under vacuum and under temperature and pressureconditions which provide for its sublimation and its migration in thevapour state onto the support. The sublimation can be carried out at atemperature ranging from 20 to 300° C., in particular from 50 to 150°C., under vacuum.

It is also possible to carry out grafting by bringing into contact in aliquid medium or in solution. In this case, the precursor can bedissolved in an organic solvent, such as pentane or ethyl ether, so asto form a homogeneous solution, and the support can subsequently besuspended in the solution comprising the precursor or by any othermethod providing contact between the support and the precursor. Thecontacting operation can be carried out at ambient temperature (20° C.)or, more generally, at a a temperature ranging from −80° C. to +150° C.under an inert atmosphere such as nitrogen. If a portion of theprecursor has not fixed to the support, it can be removed by washing orreverse sublimation.

The process of the present invention makes it possible to considerablyimprove the yield of the “methane-olysis” reaction resulting frombringing methane into contact with the starting alkane or alkanes (I) inthe presence of the metal compound (C), the catalytic stability andactivity of which over time are greatly enhanced by virtue of theapplication of a particularly high methane pressure.

The following examples illustrate the present invention and show thescale of the improvement in the reaction for the “methane-olysis” ofalkanes.

EXAMPLE 1 Preparation of a Metal Compound (C) Based on Tantalum HydrideSupported on and Grafted to Silica

A tantalum compound (C) supported on and grafted to silica was preparedin the following way.

In a first stage, 5 g of a silica, previously dehydrated and treated at500° C., and then 20 ml of an n-pentane solution comprising 800 mg (1.72millimol of tantalum) of tris(neopentyl)neopentylidenetantalum, used asprecursor and corresponding to the general formula:Ta[—CH₂—C(CH₃)₃]₃[═CH—C(CH₃)₃]  (5)were introduced under an argon atmosphere into a glass reactor, whichprecursor, by reacting at 25° C. with the hydroxyl groups of the silica,was grafted to the silica. The excess unreacted precursor was removed bywashing with n-pentane. The resulting solid compound, which constitutedthe organometallic compound grafted to the silica and which correspondedto the general formula:(Si—O)_(1.35)—Ta[═CH—C(CH₃)₃][—CH₂—C(CH₃)₃]_(1.65)was then dried under vacuum.

In a second stage, the tantalum compound, thus supported on and graftedto the silica, was subsequently treated under an atmosphere 80 kPa ofhydrogen at a temperature of 250° C. for 15 hours. By hydrogenolysis ofthe neopentyl and neopentylidene ligands, a tantalum compound (C)supported on and grafted to silica was formed which, per 100 parts bymoles of tantalum, comprised:

-   -   72 parts by moles of a tantalum hydride grafted to the silica in        the form (A) corresponding to the general formula:        [(silica support)-Si—O]₂—Ta—H  (6)        and    -   28 parts by moles of a tantalum compound grafted to the silica        in the form (B) corresponding the general formula:        [(silica support)-Si—O]₃—Ta  (7)

EXAMPLES 2 TO 5 Reaction of Methane with Propane (or “Methane-olysis” ofPropane)

A mixture comprising, per 10⁶ mol of methane, 8×10² mol of propane wascontinuously passed, at a flow rate of 1.5 ml/min, successively underthe following methane partial pressures: 0.0645 MPa (Comparative Example2), 0.5 MPa (Example 3), 125 MPa (Example 4) and 5 MPa (Example 5),through a reactor with a capacity of 5 ml which was heated to 250° C.and which comprised 300 mg of the tantalum compound (C) prepared inExample 1 (59.3 micromol of active tantalum in the form (A)).

Bringing methane into contact with propane in the presence of thetantalum compound (C) resulted in the formation of ethane according to apropane “methane-olysis” reaction which is written according thefollowing equation:C₃H₈+CH₄→2C₂H₆  (8)For each example, the instantaneous percentage of conversion of thepropane at various points in the reaction was measured and calculated,as was the number of moles of methane incorporated per mole of tantalum(in form (A)) after reacting for 3 600 minutes. The results of thesemeasurements and calculations were collated in Table 1.

TABLE 1 Number of % of % of mols of meth- Meth- conversion of conversionof ane incor- ane the propane the propane porated per partial withreaction with reaction mol of Ta after pressure for 360 for 3600reacting for Example (MPa) minutes minutes 3600 minutes 2 (compara-0.0645 10%  0% 0.01 tive) 3 0.5 98% 16% 0.19 4 1.25 99% 62% 1.58 5 5 99%91% 2.67

Table 1 showed that the percentage of conversion of the propane in thepropane “methane-olysis” reaction was greater in proportion anddecreased less over time in proportion as the methane partial pressureapplied increased. It was thus observed that the tantalum compound (C)had a catalytic activity which was remarkably stable over time, when themethane pressure applied to the “methane-olysis” reaction was high. InExample 5, it was found that the amount of butane formed was negligibleso that the process carried out under these conditions essentiallyinvolved a propane “methane-olysis” reaction.

EXAMPLES 6 AND 7 Reaction of Methane with Propane at 250° C. and 300° C.

A propane “methane-olysis” reaction (Example 6) was carried out exactlyas in Example 5, in particular at a temperature of 250° C., and anotherpropane “methane-olysis” reaction (Example 7) was also carried outexactly as in Example 5, except the fact that the temperature of thereaction was 300° C. (instead of 250° C.).

In the reaction carried out at 250° C. (Example 6) it was observed thatwhen 3 moles of propane were consumed per mole of tantalum,simultaneously 5.08 moles of ethane were produced. In the reactioncarried out at 300° C. (Example 7), when 3 moles of propane wereconsumed at 300° C. per mole of tantalum, simultaneously 5.53 moles ofethane were produced.

These results showed that when a higher temperature was used in a“methane-olysis” reaction, ethane was produced in higher amounts.

EXAMPLE 8 Reaction of Methane with n-butane (or n-butane“Methane-olysis”)

An n-butane “methane-olysis” reaction was carried out as in Example 5,except that, instead of passing a mixture of methane and propane intothe reactor, a mixture comprising, per 10⁶ mol of methane, 5.3×10² molof n-butane, with a methanol partial pressure equal to 5 MPa, was passedtherein continuously.

It was observed that, under these conditions, the n-butane“methane-olysis” reaction resulted in the simultaneous formation ofethane and propane, that the percentage of conversion of the n-butaneremained at a high value after a long reaction time and that thetantalum compound (C) deactivated relatively slowly over time.

EXAMPLE 9 Preparation of a Metal Compound (C) Based on Tungsten HydrideSupported on and Grafted to Silica

A tungsten compound (C) supported on and grafted to silica was preparedprecisely as in Example 1, except that, in the first stage, instead ofusing a solution of tris(neopentyl)neopentylidenetantalum in n-pentaneas precursor, a solution of tris(neopentyl)neopentylidenetungsten inn-pentane, corresponding to the general formula:W[—CH₂—C(CH₃)₃]₃[≡C—C(CH₃)₃]  (9)was used and that, in the second stage, instead of carrying out thehydrogenolysis at 250° C., it was carried out at 150° C. A tungstencompound (C) supported on silica, essentially in the form (A) of atungsten hydride, was thus obtained.

EXAMPLE 10 Reaction of Methane With Propane (or Propane“Methane-olysis”)

A propane “methane-olysis” reaction was carried out as in Example 5,except that, instead of using the tantalum compound prepared in Example1, the tungsten compound prepared in Example 8 was used.

It was observed that the “methane-olysis” reaction resulted in theformation of ethane, that the percentage of conversion of the propaneremained at a high value after a long reaction time and that thetungsten compound deactivated relatively slowly over time.

EXAMPLES 11 AND 12 Reaction of Methane with Propane at 375° C. (or“Methane-olysis” of Propane)

A propane “methane-olysis” reaction (Example 11) was carried out exactlyas in Example 5, except the fact that the temperature of the reactionwas 375° C. (instead of 250° C.) and that the mixture of methane andpropane continuously passed through the reactor comprised 10⁴ mol ofpropane per 10⁶ mol of methane.

A propane “methane-olysis” reaction (Example 12) was carried out exactlyas in Example 5, except the fact that the temperature of the reactionwas 375° C. (instead of 250° C.) and that the mixture of methane andpropane continuously passed through the reactor comprised 10⁵ mol ofpropane per 10⁶ mol of methane.

In the two reactions, ethane was produced in great amounts.

1. Process for the manufacture of alkanes comprising a catalyticreaction resulting from bringing methane into contact with at least oneother starting alkane (I) in the presence of a metal compound (C)capable of catalysing a reaction for the splitting and/or recombinationof a carbon-carbon bond and/or of a carbon-hydrogen bond and/or of acarbon-metal bond, which catalytic reaction results in the formation ofat least one final alkane (II) having a number of carbon atoms equal toor greater than 2, which process comprises carrying out the contactingoperation under a methane partial pressure equal to or greater than 0.1MPa.
 2. Process according to claim 1, wherein the methane partialpressure is chosen within a range from 0.1 to 100 MPa.
 3. Processaccording to claim 1, wherein the contacting operation is carried out ata temperature ranging from −30° C. to +500° C.
 4. Process according toclaim 1, wherein the contacting operation is carried out in a molarratio of methane to starting alkane(s) (I) ranging from 0.1:1 to 10⁵:1.5. Process according to claim 4, wherein the contacting operation iscarried out in a molar ratio of methane to starting alkane(s) (I)ranging from 60:1 to 10⁵:1.
 6. Process according to claim 1, wherein itcomprises contacting methane with a mixture of two or more startingalkanes (I).
 7. Process according to claim 1, wherein the startingalkane (I) is either a substituted or unsubstituted acyclic alkane or asubstituted cyclic alkane.
 8. Process according to claim 7, wherein thestarting alkane (I) is either a substituted or unsubstituted acylicalkane corresponding to the general formula:C_(n)H_(2n+2) in which n is an integer ranging from 2 to 60, or asubstituted cyclic alkane corresponding to the general formula:C_(n)H_(2n) in which n is an integer ranging from 5 to
 60. 9. Processaccording to claim 1, wherein the metal compound (C) comprises at leastone metal atom, Me, selected from the group consisting of lanthanides,the actinides and the metals from Groups 2 to 12of the Periodic Table ofthe Elements.
 10. Process according to claim 9, wherein the metal atom,Me, is at least one metal selected from the group consisting ofscandium, yttrium, lanthanum, titanium, zirconium, hafnium, vanadium,niobium, tantalum, chromium, molybdenum, tungsten, ruthenium, palladiumplatinum, iridium, cerium and neodymium.
 11. Process according to claim1, wherein the metal compound (C) is selected from the group consistingof metal compounds supported on and dispersed over a solid support,metal compounds supported on and grafted to a solid support, andnon-supported metal compounds.
 12. Process according to claim 11,wherein the supported and grafted metal compounds comprise a solidsupport to which are grafted one or more different or identical metalatoms, Me, bonded to the support via single or multiple bonds. 13.Process according to claim 12, wherein the metal atom, Me, supported onand grafted to the solid support, is bonded to at least one hydrogenatom and/or to at least one hydrocarbon radical via a carbon-metalsingle, double or triple bond.
 14. Process according to claim 13,wherein the hydrocarbon radical is a saturated or unsaturatedhydrocarbon radical having from 1 to
 20. 15. Process according to claim13, wherein the hydrocarbon radical is selected from the groupconsisting of alkyl, alkylidene, alkylidyne, aryl, aralkyl, aralkylideneand aralkylidyne radicals.
 16. Process according to claim 1, wherein themetal compound (C) is selected from the group consisting of metalhydrides and organometallic compounds.
 17. Process according to claim11, wherein the metal atom, Me, of the supported and grafted metalcompound has a degree of oxidation ranging from 1 to its maximum value.18. Process according to claim 1, wherein it is carried outcontinuously.
 19. Process according to claim 1, wherein the metalcompound (C) is selected from the group consisting of metal hydrides andorganometallic compounds of the metal Me, supported on and grafted to asolid support.
 20. Process according to claim 1, wherein the methanepartial pressure is within the range of 0.3 to 9.5 MPa.
 21. Process forincreasing the catalytic activity and stability of a metal compound (C)capable of catalysing a reaction for the splitting and/or recombinationof a carbon-carbon bond and/or of a carbon-hydrogen bond and/or of acarbon-metal bond, which compound is employed in a catalytic reactionwhich results from bringing methane into contact with at least one otherstarting alkane (I) and which results in the formation of at least onefinal alkane (II) having a number of carbon atoms equal to or greaterthan 2, which process carrying out the contacting operation under amethane partial pressure equal to or greater than 0.1 MPa.
 22. Processaccording to claim 21, wherein the methane partial pressure is withinthe range from 0.1 to 100 MPa.
 23. Process according to claim 21,wherein the methane partial pressure is chosen within a range from 0.3to 9.5 MPa.